EFFECTS OF MELT INTERCONNECTIVITY AND COMPOSITION OF THE STRENGTH OF A FINE-GRAINED GNEISS

Partial melting of crustal rocks leads to significant weakening and is thought to be one of the processes that leads to destabilization and extrusion of the mid to lower crust in collisional orogens. However, partial melting in experimentally deformed rocks almost always causes brittle behavior, which is not consistent with the processes expected at high temperatures deep in Earth’s crust. However, many of these experiments were performed at low confining pressures which inhibits crystal plastic deformation of the framework phases in the rocks. In order to analyze how melt interconnectivity develops at conditions that promote crystal plastic deformation of rock framework phases, cylinders of a fine-grained, mid-crustal foliated gneiss with foliation orientated at both 90° and 45° to the compression direction were deformed at four different temperatures (T= 850°C, 900°C, 950°C, and 1000°C) and similar confining pressure (1.5 GPa), strain rate (2*10^-6/s), and total strain (30%) using the D-DIA deformation apparatus at Argonne National Laboratory. Melt concentration increases with increasing temperatures (<1% at 850°C and 5.5% at 1000°C). At all conditions, strengths of cylinders of both orientations are almost identical. At T = 850-950°C, melt is produced only by wet melting of feldspar and quartz. At T = 1000°C, melt is primarily produced by a reaction between biotite, plagioclase and quartz, though wet melts are also observed. At the lower temperatures, melt is present in pockets parallel to the compression direction along grain boundaries and crosscutting through individual grains. At 1000°C, melts formed by the biotite reaction are observed in very long and thin pockets along grain boundaries perpendicular to the compression direction; whereas, wet melts are observed only in pockets parallel to the compression direction. Both of these melts are high silica melts, but the melt formed by the biotite reaction contains significant iron concentrations. Iron has been demonstrated to decrease the viscosity of feldspathic melts by orders of magnitude. Our results indicate that: 1) foliation orientation does not affect rock strength during partial melting and 2) that granitic melts with higher iron contents will become mobile at lower melt fractions than granitic melts with low iron contents.